1. Field of the Invention
This invention relates to novel bioactive compounds, methods of diagnostic imaging using radiolabeled compounds, and methods of making radiolabeled compounds.
2. Background Art
Alzheimer""s disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline, irreversible memory loss, disorientation, and language impairment. Postmortem examination of AD brain sections reveals abundant senile plaques (SPs) composed of amyloid-xcex2 (Axcex2) peptides and numerous neurofibrillary tangles (NFTs) formed by filaments of highly phosphorylated tau proteins (for recent reviews and additional citations see Ginsberg, S. D., et al., xe2x80x9cMolecular Pathology of Alzheimer""s Disease and Related Disorders,xe2x80x9d in Cerebral Cortex. Neurodegenerative and Age-related Changes in Structure and Function of Cerebral Cortex, Kluwer Academic/Plenum, New York (1999), pp. 603-654; Vogelsberg-Ragaglia, V., et al, xe2x80x9cCell Biology of Tau and Cytoskeletal Pathology in Alzheimer""s Disease,xe2x80x9d Alzheimer""s Disease, Lippincot, Williams and Wilkins, Philadelphia, P.A. (1999), pp.359-372). Familial AD (FAD) is caused by multiple mutations in the A precursor protein (APP), presenilin 1 (PS1) and presenilin 2 (PS2) genes (Ginsberg, S. D., et al., xe2x80x9cMolecular Pathology of Alzheimer""s Disease and Related Disorders,xe2x80x9d in Cerebral Cortex: Neurodegenerative and Age-related Changes in Structure and Function of Cerebral Cortex, Kluwer Academic/Plenum, New York (1999), pp. 603-654; Vogelsberg-Ragaglia, V., et al., xe2x80x9cCell Biology of Tau and Cytoskeletal Pathology in Alzheimer""s Disease,xe2x80x9d Alzheimer""s Disease, Lippincot, Williams and Wilkins, Philadelphia, P.A. (1999), pp. 359-372).
While the exact mechanisms underlying AD are not fully understood, all pathogenic FAD mutations studied thus far increase production of the more amyloidogenic 42-43 amino-acid long form of the Axcex2 peptide. Thus, at least in FAD, dysregulation of Axcex2 production appears to be sufficient to induce a cascade of events leading to neurodegeneration. Indeed, the amyloid cascade hypothesis suggests that formation of extracellular fibrillar Axcex2 aggregates in the brain may be a pivotal event in AD pathogenesis (Selkoe, D. J., xe2x80x9cBiology of B-amyloid Precursor Protein and the Mechanism of Alzheimer""s Disease,xe2x80x9d Alzheimer""s Disease, Lippincot Williams and Wilkins, Philadelphia, P.A. (1999), pp. 293-310; Selkoe, D. J., J. Am. Med. Assoc. 283:1615-1617 (2000); Naslund, J., et al., J. Am. Med. Assoc. 283:1571-1577 (2000); Golde, T. E., et al., Biochimica et Biophysica Acta 1502:172-187 (2000)).
Various approaches in trying to inhibit the production and reduce the accumulation of fibrillar Axcex2 in the brain are currently being evaluated as potential therapies for AD (Skovronsky, D. M. and Lee, V. M., Trends Pharmacol. Sci. 21:161-163 (2000); Vassar, R., et al., Science 286:735-741 (1999); Wolfe, M. S., et al., J. Med. Chem. 41:6-9 (1998); Moore, C. L., et al., J. Med. Chem. 43:3434-3442 (2000); Findeis, M. A., Biochimica et Biophysica Acta 1502:76-84 (2000); Kuner, P., Bohrmann, et al., J. Biol. Chem. 275:1673-1678 (2000)). It is therefore of great interest to develop ligands that specifically bind fibrillar Axcex2 aggregates. Since extracellular SPs are accessible targets, these new ligands could be used as in vivo diagnostic tools and as probes to visualize the progressive deposition of Axcex2 in studies of AD amyloidogenesis in living patients.
To this end, several interesting approaches for developing fibrillar Axcex2 aggregate-specific ligands have been reported (Ashburn, T. T., et al., Chem. Biol. 3:351-358 (1996); Han, G., et al., J. Am. Chem. Soc. 118:4506-4507 (1996); Klunk, W. E., et al., Biol. Psychiatry 35:627 (1994); Klunk, W. E., et al., Neurobiol. Aging 16:541-548 (1995); Klunk, W. E., et al., Society for Neuroscience Abstract 23:1638 (1997); Mathis, C. A., et al., Proc. XIIth Intl. Symp. Radiopharm. Chem., Uppsala, Sweden: 94-95 (1997); Lorenzo, A. and Yankner, B. A., Proc. Natl. Acad. Sci. US.A. 91:12243-12247 (1994); Zhen, W., et al., J. Med. Chem. 42:2805-2815 (1999)). The most attractive approach is based on highly conjugated chrysamine-G (CG) and Congo red (CR), and the latter has been used for fluorescent staining of SPs and NFTs in postmortem AD brain sections (Ashburn, T. T., et al., Chem. Biol. 3:351-358 (1996); Klunk, W. E., et al., J. Histochem. Cytochem. 37:1273-1281 (1989)). The inhibition constants (Ki) for binding to fibrillar Axcex2 aggregates of CR, CG, and 3xe2x80x2-bromo- and 3xe2x80x2-iodo derivatives of CG are 2,800, 370, 300 and 250 nM, respectively (Mathis, C. A., et al., Proc. XIIth Intl. Symp. Radiopharm. Chem., Uppsala, Sweden: 94-95 (1997)). These compounds have been shown to bind selectively to Axcex2 (1-40) peptide aggregates in vitro as well as to fibrillar Axcex2 deposits in AD brain sections (Mathis, C. A., et al., Proc. XIIth Intl. Symp. Radiopharm. Chem., Uppsala, Sweden: 94-95 (1997)).
Amyloidosis is a condition characterized by the accumulation of various insoluble, fibrillar proteins in the tissues of a patient. An amyloid deposit is formed by the aggregation of amyloid proteins, followed by the further combination of aggregates and/or amyloid proteins.
In addition to the role of amyloid deposits in Alzheimer""s disease, the presence of amyloid deposits has been shown in diseases such as Mediterranean fever, Muckle-Wells syndrome, idiopathetic myeloma, amyloid polyneuropathy, amyloid cardiomyopathy, systemic senile amyloidosis, amyloid polyneuropathy, hereditary cerebral hemorrhage with amyloidosis, Down""s syndrome, Scrapie, Creutzfeldt-Jacob disease, Kuru, Gerstamnn-Straussler-Scheinker syndrome, medullary carcinoma of the thyroid, Isolated atrial amyloid, xcex22-microglobulin amyloid in dialysis patients, inclusion body myositis, xcex22-amyloid deposits in muscle wasting disease, and Islets of Langerhans diabetes Type II insulinoma.
Thus, a simple, noninvasive method for detecting and quantitating amyloid deposits in a patient has been eagerly sought. Presently, detection of amyloid deposits involves histological analysis of biopsy or autopsy materials. Both methods have drawbacks. For example, an autopsy can only be used for a postmortem diagnosis.
The direct imaging of amyloid deposits in vivo is difficult, as the deposits have many of the same physical properties (e.g., density and water content) as normal tissues. Attempts to image amyloid deposits using magnetic resonance imaging (MRI) and computer-assisted tomography (CAT) have been disappointing and have detected amyloid deposits only under certain favorable conditions. In addition, efforts to label amyloid deposits with antibodies, serum amyloid P protein, or other probe molecules have provided some selectivity on the periphery of tissues, but have provided for poor imaging of tissue interiors.
Potential ligands for detecting Axcex2 aggregates in the living brain must cross the intact blood-brain barrier. Thus brain uptake can be improved by using ligands with relatively smaller molecular size (compared to Congo Red) and increased lipophilicity. Highly conjugated thioflavins (S and T) are commonly used as dyes for staining the Axcex2 aggregates in the AD brain (Elhaddaoui, A., et al., Biospectroscopy 1: 351-356 (1995)). These compounds are based on benzothiazole, which is relatively small in molecular size. However, thioflavins contain an ionic quarternary amine, which is permanently charged and unfavorable for brain uptake.
Thus, it would be useful to have a noninvasive technique for imaging and quantitating amyloid deposits in a patient. In addition, it would be useful to have compounds that inhibit the aggregation of amyloid proteins to form amyloid deposits and a method for determining a compound""s ability to inhibit amyloid protein aggregation.
The present invention provides novel compounds of Formula I, II, III or IIIxe2x80x2 that bind preferentially to amyloid aggregates.
The present invention also provides diagnostic compositions comprising a radiolabeled compound of Formula I, II, III or IIIxe2x80x2, and a pharmaceutically acceptable carrier or diluent.
The invention further provides a method of imaging amyloid deposits, the method comprising introducing into a patient a detectable quantity of a labeled compound of Formula I, II, III or IIIxe2x80x2 or a pharmaceutically acceptable salt, ester, amide, or prodrug thereof.
The present invention also provides a method for inhibiting the aggregation of amyloid proteins, the method comprising administering to a mammal an amyloid inhibiting amount of a compound Formula I, II, III or IIIxe2x80x2 or a pharmaceutically acceptable salt, ester, amide, or prodrug thereof.
A further aspect of this invention is directed to methods and intermediates useful for synthesizing the amyloid inhibiting and imaging compounds of Formula I, II, III or IIIxe2x80x2 described herein.